MEMS devices are used for numerous applications where it is required to have moving parts located within a sealed cavity. It is known to make such devices by providing a base which consists of an etched SOI (silicon-on-insulator) substrate. A lid is then applied to the base to make a hermetically sealed device with a cavity inside. A typical example of such a device is an RF MEMS device. The Q-factor of such a device will degrade as the pressure within the sealed cavity rises above 10 Pascals due to leakage.
Some way must be found to provide an effective seal between the base, also known as the wafer level package, and the lid. It is known to provide a sealing ring in the form of a glass frit ring extending around the cavity portion of the package and hermetically seal the lid to the frit ring. In order to provide the necessary lateral seal, the frit must be in the order of 1 mm wide. This width limits the number of devices that can be made on one wafer, and hence determines the cost of the device.
In order to reduce the width of the sealing ring, it has been proposed to replace the glass frit ring with a metal sealing ring. The metal ring has the advantage that it is highly impermeable and thus enables the width of the sealing ring to be substantially reduced. Typically, with a metal ring present the sealing ring can be reduced from 1 mm to about 40 μm. The problem, however, is that if a metal ring is used as part of the sealing ring, it is not easy to take the necessary electrical leads outside the sealed cavity because if they are laid on top of the substrate they will be shorted by the metal ring.